Malnutrition in the form of iodine deficiency is "the most common preventable cause of mental impairment worldwide." Even moderate iodine deficiency, especially in pregnant women and infants, lowers intelligence by 10 to 15 I.Q. points, shaving incalculable potential off a nation’s development. The most visible and severe effects — disabling goiters, cretinism and dwarfism — affect a tiny minority, usually in mountain villages. But 16 percent of the world’s people have at least mild goiter, a swollen thyroid gland in the neck.
Iron deficiency anemia in children under two years of age likely affects brain function acutely and probably also chronically. Folate deficiency has been linked to neural tube defects.
Protein-calorie malnutrition can cause cognitive impairments. For humans, "critical period varies from the final third of gestation to the first 2 years of life".
Nutritional supplement treatment may be appropriate for major depression, bipolar disorder, schizophrenia, and obsessive compulsive disorder, the four most common mental disorders in developed countries. Supplements that have been studied most for mood elevation and stabilization include eicosapentaenoic acid and docosahexaenoic acid (each of which are an omega-3 fatty acid contained in fish oil, but not in flaxseed oil), vitamin B12, folic acid, and inositol.